A non-contact type power supply unit is characterized by transferring electric power from a primary side to a secondary side, and since the primary side circuit and secondary side circuit are packaged in separate enclosures that are completely independent from each other, it is very important to establish magnetic coupling between a primary transformer and a secondary transformer of the non-contact type power supply unit for efficient transfer of electric power. In order to achieve an improvement in the magnetic coupling, use of E--E cores is considered effective. Also, it has been a usual practice to make the opposing area between the primary coil and the secondary coil of the transformers as large as possible for better magnetic coupling.
An explanation will be given to prior art technologies in the following with reference to FIG. 8 to FIG. 10.
FIG. 8 shows a structure wherein E type cores are used as a primary core 1 and also a secondary core 5. A primary coil 2 is placed in the slot of the primary core 1 (E type core), thereby forming a primary transformer 4. Also, a secondary coil 6 is placed in the slot of the secondary core 5 (E type core), thereby forming a secondary transformer 8. Reference numerals 3 and 7 indicate a primary bobbin and a secondary bobbin, respectively.
Next, how the foregoing structure works will be explained. When high frequency currents flow in the primary coil 2 formed of windings wound concentrically, magnetic fluxes are produced in the primary coil 2 and primary core 1. The foregoing magnetic fluxes are propagated to the secondary core 5 and secondary coil 6 and converted to electric power, which is then supplied to load.
FIG. 9 shows a structure wherein the opposing area between the primary coil 2 and the secondary coil 6 is made large in order to intensify the magnetic coupling between windings.
Also, in order to increase the foregoing magnetic coupling, an arrangement is made to dispose the secondary coil 6 and the primary coil 2 concentrically as shown in FIG. 10(a).
Further, there has been provided a structure, wherein a mechanical switch 8a and a magnetic switch 9a are used together in order to enable the primary coil to tell whether what receives electric power from the primary coil is the secondary coil or some other foreign objects and also to reduce a supply of electric power to the foreign objects, thereby preventing an abnormal heating of the foreign objects from taking place, and when a magnetic field generator 10a installed on a secondary enclosure 5a comes close to the magnetic switch 9a, the magnetic switch 9a is turned on and even when the mechanical switch 8a installed on a primary enclosure 4a is turned on by an object other than the secondary enclosure 5a, an arrangement is made so that electric power is not applied to the primary coil, as shown in FIG. 10(b).
However, with the structure of FIG. 8, wherein open magnetic circuits are formed, the magnetic fluxes from the middle leg 9 of the primary core 1 (E type core) are returned to the outer legs 10 by short circuiting, thus resulting in a reduction of magnetic fluxes that are propagated to the secondary side and ending up with a poor efficiency.
On the other hand, with the structure of FIG. 9, wherein the opposing area between both coils is made large, the dimensions of the primary side and secondary side including the enclosures become extremely large, thereby hindering a unit, wherein this structure is incorporated, from achieving a downsizing.
When the secondary coil 6 and the primary coil 2 are disposed concentrically as shown in FIG. 10(a), the magnetic coupling between the primary coil 2 and the secondary coil 6 is intensified, resulting in an increase of the transmission of electric power. However, the diameter of the primary coil 2 becomes the inner diameter of the secondary coil 6 resulting in the problems of an increase of dimensions of the secondary coil 6 and the like.
Furthermore, when the mechanical switch 8a is provided on the primary enclosure 4a to prevent an abnormal heating of foreign objects as shown in FIG. 10(b), electrical contacts once eliminated by employing a non-contact structure are used again, thereby bringing about the danger of failure in contacts and also making it difficult to make the direct current power supply unit in a hermetically sealed construction or a water-proof construction. In addition, use of the magnetic switch 9a such as a Hall effect device and the like has caused such problems as a difficulty for the magnetic switch 9a to tell distinctly the difference between a heat producing object other than the secondary enclosure 5a when the foregoing object is made of a magnetic substance and the like and the secondary transformer that includes the secondary enclosure 5a, and the like.
The present invention provides an inexpensive and yet reliable power supply unit whereby the shortcomings of the prior art as described in the above are eliminated, a simpler structure is realized, electric power transmission efficiencies are enhanced and a downsizing of the equipment is made possible.